This invention relates to steam sterilization indicators. More particularly, it relates to steam sterilization indicators which may be variable and/or adjustable in rate of indication at different sterilization temperatures.
Hospital utensils, such as surgical instruments, undergo sterilization for each use. In most instances, an autoclave is used to expose the utensils to live steam at various temperatures, usually between 250.degree. F. and 275.degree. F., although other temperatures are also used. The purpose for providing such sterilization is to destroy, with a high probability of success of safety factor, the microbial contamination which may be contained on these utensils. It is important to gauge the sterilization process so that the user may be assured that the utensils have, in fact, been subjected to those well-defined conditions necessary to render the material free of living organisms with a high probability of success. Several devices and techniques have been used to provide for such indication.
Of course, the materials which have been processed through the sterilizer could be biologically sampled to determine biological activity. However, this technique, while highly accurate, obviously would be very costly and impractical.
One very reliable method for providing indication of sterility is to utilize challenge spores which are placed in the autoclave during sterilization and then examined for their biological activity afterwards. For steam sterilizations, these challenge spores are usually Bacillus stearothermophilus and are used because they have a very high resistance to steam sterilization, thus giving a large safety factor. One example of this technique is set forth in U.S. Pat. No. 3,440,144, which provides a device for conducting such a test without the need to worry about subsequent contamination after the sterilization process is completed.
Another means to indicate sterilization is the use of sterilizer temperature recorder and gauges. These devices are usually attached to the sterilizer and measure the temperature in the sterilizer's exhaust line. While they are able to detect most malfunctions of the sterilizer, they cannot measure the condition at the place where the instruments were being sterilized.
A means for measuring the presence of steam, which is critical for steam sterilization, in an autoclave indicating tape. An example of such indicator tape is set forth in U.S. Pat. No. 2,889,799. A pressure-sensitive adhesive tape is used which includes a heat modifiable dye stuff impregnated thereon changing color at predetermined temperatures. However, these indicator tapes do not take into account the time that the instruments have been exposed to sterilizing temperature, and furthermore, are susceptible to prematurely changing color at low temperatures.
Another test which has been utilized is a so-called Bowie and Dick test. This test measures the uniformity of steam concentration in dressing packs. The test consists of several strips of autoclave indicating tape on a sheet of paper which is placed in the test pack. The tape on the paper is measured for uniformity of color change. One of the major limitations of this test is its failure to distinguish between high temperatures for a short period of time or low temperatures for a long period of time.
More recently, steam sterility indicators have been provided which integrate time, temperature and steam presence. Such a device is shown in U.S. Pat. No. 3,981,683. This device utilized a chemical such as 2-ethoxybenzamide or salicylamide as a fusible material. The melting points of these compounds are depressed by the presence of steam. A wicking strip is provided in close proximity to the chemical so that upon melt the chemical will slowly travel up the wick at a rate proportional to the sterilization temperature and time of exposure to such temperature, as well as the presence of steam. The device includes a cover strip which is a polymeric rate controlling film permitting water vapor (steam) to pass through thus depressing the melting point of the chemical. The strip cover and the wick are adhered to a backing by the use of an adhesive such as a silicone.
The device set forth in U.S. Pat. No. 3,981,683 is particularly useful where the exact temperature in the sterilization process it unknown. If it were known that the apparatus to be sterilized was an exact temperature, for example 250.degree. F., then the stabilizer could be run for an exact amount of time so that the user could be assured of sterilization within a certain safety factor. However, without fitting the autoclave with some highly sophisticated and accurate monitoring equipment, it is impossible to know whether all areas of the autoclave are uniform at the same temperature. It is well known that the temperature of items being sterilized can vary due to many variables such as air entrapment, penetration of steam through packing material and position within the autoclave. Therefore, due to this unknown variable of temperature, it is a common practice for the microbiologist to investigate how a controlled change of temperature will affect the kill of the microorganism. He would do this by repeating the microbial death rate experiment at temperatures other than 250.degree. F. After completing these experiments at other temperatures, a relationship can be obtained where the amount of time required to produce say 10.sup.-5 probability of surviving microorganisms, since this or some other safety factor producing a non-sterile item can be calculated. FIG. 3 shows an example of what the relationship of kill time vs. temperature might look like. The slope of the line is typical for microbial death rates, and, as can be seen, it is highly temperature sensitive. The death rate might be slowed down by a factor of 10 with a decrease of only 18.degree. F. Conversely, an increase in temperature of only 18.degree. F., will require only 1/10 the sterilization time. In other words, sterilizing to a probability of 10.sup.-5 in the example in FIG. 3 requires a 110 minutes at 232.degree. F., 11 minutes at 250.degree. F., and 1.1 minutes at 268.degree. F. This value of 18.degree. F. has been called the Z value and is defined as the number of degrees that are required to traverse a thermal death rate curve by one log. Thus this Z value becomes important when estimating spore death at different temperatures. This relationship has been defined mathmatically through the following equation: EQU t=(F.sub..).times.10.sup.[(250-T)/Z)] EQU where t=the amount of time required at the actual process temperature (T).
In other words, it would require t minutes at temperature T in order to do the equivalent amount of sterilization as F.sub.o minutes at 250.degree. F. (the reference temperature for steam sterilization). While a Z value of 18.degree. F. is typical, it may vary quite often from between 16.degree. F. to 23.degree. F., and other values, depending on the type of microorganism, the pH and salt concentration as well as other variables. Therefore, if an adequate sterilization process is to be described, you must not only know the relative resistance at 250.degree. F., but also the relative resistance at other temperatures. Thus the Z values must be known.
The graph in FIG. 4 graphically illustrates how a change in Z value can affect the sterilization times required at temperatures other than 250.degree. F. Notice the different slopes of the lines for the various Z values. By using the standard sterilization equation set forth above, you can calculate that if Z is equal to 23.degree. F., a time of 81.5 minutes is required at 230.degree. F. to obtain an F.sub.o of 11 minutes. Conversely, for the same F.sub.o, the time would have to be increased to 195.5 minutes at 230.degree. F. for a Z value equal to 16.degree. F. Thus it may be seen that, at temperatures lower than 250.degree. F., as the Z value decreases, the kill time at predetermined temperatures increases.
The device set forth in U.S. Pat. No. 3,981,683 provides an indicator which is useful in a steam sterility process for spores having a Z value of 18.degree. F., but it is not very flexible in terms of measuring sterilization of devices contaminated with spores with other Z values. Furthermore, it is a rather long device, thus materials are wasted.